The art of electrophotographic copying is well-developed, especially in the aspect of dry toner developing. In one such method a rigid drum of photoconductive material such as amorphous selenium is surface-charged by corona, exposed to a projected light or other radiant energy to achieve an electrostatic latent image, developed by electroscopic powder and the developed powder image is transferred by pressure to a carrier medium such as plain paper. The resulting transfer is normally fused by heat, pressure or application of both. The drum is cleaned and reused many times.
In another method known as electrofax, the electrophotographic member is a prepared flexible sheet of conductive paper or the like coated with a layer of photoconductive material such as zinc oxide in an organic matrix. The sheet itself is charged, exposed and passed through a bath of liquid toner, the toner particles in the suspension being selectively deposited upon the photoconductive layer. The image is dried and fused by heat and the entire sheet becomes the copy. This form of copying has been practiced decreasingly because of the preferred demand for plain paper copiers.
More recently the electrophotographic member has taken the form of an endless belt having an outer photoconductive layer and an interior ohmic or conductive layer that is grounded. The belt is engaged between a pair of rollers that are parallel and spaced apart to suspend the reaches of the belt between them. The belt surface is charged by corona means as the belt rotates, is exposed, toned and the developed image transferred to a member of plain paper. Both dry powder toner development and liquid toner development are known. The principal advantages of belt use are savings in space where the belt loop is fairly narrow, economy of construction and ease of replacement.
Reference may be had to U.S. Pat. Nos. 4,236,807; 4,259,005; and 4,264,199 for disclosure of a plain paper copier in which a belt loop is suspended between a pair of rollers, the charging is progressively effected at one end of the bottom reach, the exposing is effected also on the bottom reach, the developing is effected at the second end of the bottom reach by liquid toner. The developed image is brought around one roller to the top reach and the transfer is effected opposite the second roller.
This invention is an improvement over the just-described method and the apparatus thereof in that certain disadvantages of the just-described method and apparatus are diminished, if not eliminated. In addition certain benefits are achieved which were not achieved in said just-described method and apparatus. Some of the disadvantages diminished and benefits achieved will be mentioned hereinafter.
The width of the area along the length of the prior belt which was subjected to application of toner at any instant was extremely narrow, normally not much more than line contact. This established a requirement for a high surface potential in order to attract as much of the toner particles as possible in the short time of application.
According to the invention, the width of the area along the length of the belt which is subjected to toner application at any instant has been increased by an order of fifty times or more thereby decreasing substantially the required surface potential and the voltage demands upon the charging apparatus because the photoconductive surface need not be charged initially as high. In the prior apparatus the belt surface had to be charged to a surface potential of about 90 volts whereas with the invention the surface potential need only be of the order of 50 to 60 volts. These potentials are merely examples and will vary for different types of photoconductors. That which was used in the example was a crystalline sputtered cadmium sulfide about 2 microns thick on an ohmic layer of suitable metal.
In other belted copier apparatus the belt had to be maintained at very high tension in order to achieve positive drive for timing purposes and taut areas for exposure and toning. This increased the expense of bearings needed to support the belt. Rollers and shafts tended to bow in their centers because of the force applied.
According to the invention the tension of the belt at the toning station is increased by a unique application of force. The belt is diverted in a tortuous bow or jog spaced inwardly of the plane of the bottom reach thereby tensioning the belt locally with great force through amplification of a small force. The remainder of the belt is taut without superfluous stress so that economical bearings may be used for the support rollers and there is no bowing or bending of shafts.
A highly simplified method of replacing the belt results from the novel structure of the invention thereby solving a difficult problem with the prior device. This is achieved with the addition of no important components but is a dividend inherent in the inventive structure.
The invention has resulted in a toner application bias that is more uniform and efficient than the bias of prior devices and which has less tendency to spark because of the uniformity and power surface potential.
The apparatus of the invention enables the achievement of smaller and more uniform gaps than previously, both at the toning station and at the transfer station.
The invention is especially useful in the circumstance that the toner suspension has a greater viscosity than normal liquid toner suspensions. For example, the liquid toner which was usually used in the electrofax devices had a consistency almost like water with perhaps a surface tension even less than water because the liquid carrier was an insulating isoparaffinic hydrocarbon solvent. Typically such solvent is one of several types manufactured by the Exxon company and known by the trademark Isopar. The solvent is sold in various viscosities designated by characters of the alphobet, the viscosity being higher for the later characters. The electrofax Isopar solvent was normally type G or H with a viscosity of 1.00 or 1.3 centipoises, respectively, at 25.degree. C. Such toner suspension would normally have 1% to 2% solids suspended therein.
A system and method have been devised which utilize the same type of hydrocarbon solvent but carrying 3% to 4% solids suspended. Such system and method are disclosed in a patent application Ser. No. 315,542 filed Oct. 27, 1981, in which the applicant herein is one the inventors and the application is owned by the assignee hereof.
The invention enables the use of a hydrocarbon carrier and diluent for the toner particles which has much higher viscosity than Isopar G or H. The hydrocarbon could have a viscosity as high as 2.46 centipoises at 25.degree. C. and is commercially available as Isopar M. This solvent is capable of carrying substantially more than 4% solids. Thus the toner is effectively more viscous, is easier to "plate" upon rollers, enables greater densities of development, throws less sediment than previous viscous toners and is less aromatic.
In previous apparatus it was found best to use metering means such as a roller, doctor blade or other device to layer the toner uniformly across the toning roller which carries the toner against the moving belt. An extracting roller and an associated doctor blade were deemed of importance to trim excessive toner deposit from the developed image after passing through the toning station.
The effectiveness of the invention is such that no metering or extracting means are required. Instead, there is a single toning roller dipped into the toner liquid and carrying the same directly to the belt without the need for any intervening means or structure modifying the layer of toner brought to the belt by that single roller. Unquestionably, the resulting toning sump and roller means are greatly simplified over prior devices while being more effective.
Many other advantages result from the invention which can be ascertained by a study of the specification which follows. For example, the velocity of the belt with the new system can be increased over that which obtained previously. Using the system of said application Ser. No. 315,542, satisfactory copying was achieved with a belt speed of ten inches per second. Using the method and apparatus of the invention this speed can be materially increased without sacrifice of density and resolution. Accordingly the imaging cycle can be shorter and copies made faster.
It will be seen that the invention is described in connection with a copier of the so-called convenience type, but the invention is not limited to that example. It could be used in any environment where a developed image is transferred from a belt to a carrier medium. Thus the invention may be applied to color proofers, plate makers, etc. and the carrier medium can be plain paper, film or the like.